Mooring system using cables and buckled riser pipe

ABSTRACT

THIS INVENTION RELATES TO A MOORING SYSTEM FOR A FLOATING STRUCTURE. SPECIAL BOUYANCY CHAMBERS SUPPORT THE STRUCTURE, INCLUDING A WORK DECK, ABOVE A BODY OF WATER. THE STRUCTURE IS CONNECTED TO ANCHORS AT THE FLOOR OF THE BODY OF WATER BY ELONGATED MEMBERS SUCH AS CABLES AND PIPES HAVING DIFFERENT ELONGATIONS PER UNIT OF STRESS. THE MAIN ANCHORING LOAD IS CARRIED BY THE CABLES AND THE PIPES ARE USED PRIMARILY IN DRILLING OPERATIONS OR FOR TRANSPORTING PETROLEUM FROM AN UNDERWATER WELL. BECAUSE OF WAVE ACTION, THE FLOATING PLATFORM IS SUBJECT TO VERTICAL HEAVE FORCES. THE MAXIMUM VERTICAL STRESS APPLIED TO THE CABLES CAUSES THEM TO ELONGATE A LENGTH $L. THE RISER PIPE IS INSTALLED UNDER SUFFICIENT COMPRESSION SO THAT IT IS IN A BUCKLED CONFIGURATION WHEN THE FLOATING STRUCTURE IS IN A NEUTRAL POSITION. THE LENGTH OF THE BUCKLED RISER PIPE EXCEEDS THE VERTICAL DISTANCE FROM THE BOUYANCY MEANS TO THE WELL HEAD BY AN AMOUNT EQUAL TO THE MAXIMUM $ OF THE CABLE. AS THE VERTICAL DISTANCE INCREASES BY $L. THE BUCKLED RISER WILL STRAIGHTEN, RATHER THAN ELONGATE, WITHOUT A CHANGE IN AXIAL FORCE. THUS THE BASIC LOAD-CARRYING MEMBER REMAINS THE CABLE. IN A PREFERRED EMBODIMENT THE CABLE IS SLIDEABLY CONNECTED TO THE POINTS ALONG THE BUCKLED RISER PIPE CORRESPONDING TO THE NODES OF THE BUCKLED CONFIGURATION.

Mar ch 23, 1971 DlxoN ET AL 3,512,272

MOORING SYSTEM USING CABLES AND BUCKLED RISER PIPE Filed March 5, 1 969 2 Sheets-Sheet 1 IO M? 1| l4 i k J r E h F KENNETH A.BLENKARN DAVlD A. DIXON INVENTORS ATTORNEY March 23,1971 Q 7 D, DlXON 'ET AL MOORING SYSTEM USING CABLES AND BUCKLED RISER PIPE Filed March 5, 1969 4 2 Sheets-Sheet 2 ISA FIG 2 KENNETH A.BLENKARN DAVID A DIXON INVENTORS W &

FIGIS ATTORNEY United States Patent Olfice 3,572,272 MOON G SYSTEM USING CABLES AND BUCKLED RISER PIPE David A. Dixon and Kenneth A. Blenkarn, Tulsa, Okla, assignors to Pan American Petroleum Corporation, Tulsa, Okla.

Filed Mar. 5, 11969, Ser. No. 804,590 Int. Cl. B63b 35/44; E02b 17/00; EZIb 15/02 US. Cl. 114l.5 8 Claims ABSCT OF THE DISCLOSURE This invention relates to a mooring system for a floating structure. Special buoyancy chambers support the structure, including a work deck, above a body of water. The structure is connected to anchors at the floor of the body of water by elongated members such as cables and pipes having different elongations per unit of stress. The main anchoring load is carried by the cables and the pipes are used primarily in drilling operations or for transporting petroleum from an underwater well. Because of wave action, the floating platform is subject to vertical heave forces. The maximum vertical stress applied to the cables causes them to elongate a length AL. The riser pipe is installed under sufficient compression so that it is in a buckled configuration when the floating structure is in a neutral position. The length of the buckled riser pipe exceeds the vertical distance from the buoyancy means to the well head by an amount equal to the maximum AL of the cable. As the vertical distance increases by AL, the buckled riser will straighten, rather than elongate, without a change in axial force. Thus the basic load-carrying member remains the cable. In a preferred embodiment the cable is slideably connected to the points along the buckled riser pipe corresponding to the nodes of the buckled configuration. I

BACKGROUND OF THE INVENTION (1) Field of the invention This invention relates to a structure floating on a body of water. More particularly, the invention relates to a system for anchoring such floating structure to the floor of the body of water. It relates to a mooring system in which the floating structure is connected to the ocean floor by essentially vertical cables and riser pipes. It relates especially to a system whereby the total anchoring load is carried essentially by the cables, and the riser pipes are used essentially for conducting fluid from the vicinity of the ocean floor, such as from subsea wells, to the floating structure.

(2) Setting of the invention In recent years there has been considerable attention attracted to the drilling and production of wells located in Water, particularly in the Gulf of Mexico and off the coast of California. Wells may be drilled in the ocean floor from either fixed platforms in relatively shallow water or from floating structures or vessels in deeper water.

In recent years there have been many different kinds of floating structures used which for the most part have been maintained on station by conventional spread-catenary mooring lines or by propulsion thruster units. One scheme, having a different anchoring system, which recently has received considerable attention is employed in the so-called vertically moored platform. One such platform is described in US. Pat. 3,154,039, issued Oct. 27, 1964. A key feature of such vertically moored platforms is that the floating structure is connected to an anchor base only by elongated, parallel members. These anchoring, elongated members are held in tension by excess buoyancy of the platform. This feature offers a remedy for one of the major 3,572,272 Patented Mar. 23, 1971 problems arising in conducting drilling or like operations from a floating structure. This major problem is that the response of ordinary-type barges to ocean waves may exert a substantial amount of vertical heave or roll motion. Such excess motion significantly hinders drilling operations. Some of these problems are eliminated or at least reduced in the vertically moored platforms. Being subjected to tension, the elongated, parallel members of the vertically moored platform are extensible only to a small extent and thus restrain the platform to move primarily in a horizontal direction. This virtually eliminates heave and roll motion. The co-pending application, now abandoned Ser. No. 754,628, filed Aug. 22, 1968, in the name of Kenneth A. Blenkarn, describes a system which eliminates or minimizes forces imposed on a floating structure, such as those caused by passing waves. However, even under the more ideal conditions, it is anticipated that there will be changes in vertical forces which will cause elongation of the cables of at least about a foot in 300-foot water, for example. While this is not a great deal of elongation, if the mooring lines are both cables and riser pipes, this can cause considerable difficulty in load distribution between the two.

BRIEF DESCRIPTION OF THE INVENTION This invention concerns a system in which the floating structure supported on the body of water is connected to the anchor base by essentially vertical, parallel members including both cables and pipes. In this system the main anchoring load is carried by the cables and the pipes are used primarily for conveying petroleum from the subsea well to the surface. The structure has a neutral position which is meant to be the position it would have in still water. The structure is subject to vertical motion which increases the vertical force on the cable. This increased force on the cable will cause it to elongate a known or determinable maximum amount AL. The riser pipe is installed such that when the structure is in its neutral position the riser pipe is in a buckled configuration. The amount of buckling is sufficient so that the length of the buckled riser pipe exceeds the normal vertical dimension between the structure and the well head or anchor by an amount equal to the maximum anticipated elongation of the cables. In this manner the anchoring load will nearly all be carried by the cables. It is further preferred that the cable be slideably attached to the buckled riser pipe so that the dynamic motion of the riser pipes will be restrained through composite action with the cables. HOW- ever, these slideable attachments should be placed at the nodes of the buckled riser configuration in order not to restrict buckling.

DRAWINGS Various objects and a better understanding of the invention can be had from the following description taken in conjunction with the drawings in which:

FIG. 1 illustrates anchor cables and riser pipes extending downward from one leg of a vertically moored platform to anchor means;

FIG. 2 illustrates one embodiment of the connection of the upper end of the cables and riser pipes to the floating structure;

FIG. 3 illustrates another means of connecting the upper end of the cables and riser pipes to the floating structure.

Attention is first directed to FIG. 1. Shown thereon is a structure 10 supported above the surface of a body of water 12 by vertically oriented buoyancy means or chambers 14. There is only one buoyancy chamber 14 shown; however, there are ordinarily three o four such buoyancy chambers. For a discussion of a preferred form of such buoyancy chambers, attention is directed to the co-pending application Ser. No. 754,628 supra. In this invention the principal tethering or anchoring device is cables. They are illustrated in the drawing as cables 16 and 18, which anchor buoyancy chamber 14 to a base template anchored to the ocean floor 22. However, even though the cables 16 and 18 are the main anchoring devices, it is nevertheless necessary that there be a certain number of pipe risers containing flow lines to carry production from the submerged oil wells to the platform 10. Accordingly, there is illustrated in the drawing a flow line riser 24 which extends from submerged well head 26 to within buoyancy chamber 14. It will be noted that flow line riser 24 is buckled. The purpose and amount of buckling needed will be discussed further hereinafter. It is sufficient to say at this time that the pipe is buckled and has nodes along the length thereof. These node points can be calculated and appear, for example, at points 28, 30 and 32. At each such node the flow line riser 24 is slideably connected to the cables 16 and 18. These can be in the form of centralizers 34, 36 and 38. Each such centralizer is securely fixed to the riser pipe 24 but is slideably mounted on the cables 16 and 18.

By providing these centralizers 34, 36 and 38 we cause the system to behave laterally dynamically as if the same tension were applied directly to the riser pipe as applied to the cables but at the same time the riser pipe is not bearing suflicient tension to be considered a principal anchoring or tethering member.

As will be explained more fully, the flow line riser 24 is installed in a buckled position. If the fiow line riser 24 were not installed in a buckled position, then a combined riser pipe-cable system would have a serious disadvantage which would arise from the extensional stiifness of the riser pipe relative to the cable. Without our invention, a vertical heave on the vessel will cause most of the additional stress caused by such vertical heave to be taken up by the pipe. This is so because cables and riser pipes ordinarily have different elongations for the same stress. Conversely, for a given change in length, the riser pipe will experience a relatively greater change in load. In our system we want the anchoring load to be carried nearly completely by the cables. In our system this is accomplished.

We solve this problem thus raised by initially installing a riser 2-1 in a buckled position. When installing the device, the cables 16 and 18 are made sufficiently taut to hold a buoyancy means 14 in a neutral position 40 in still water. We determine what we can anticipate as a maximum vertical force on cables 16 and 18, taking into account the size of the buoyancy chambers 14, the load being carried and the expected Waves and currents to be encountered. From this maximum vertical force we determine the expected maximum elongation of the cables 16 and 18 if they are the primary load-carrying members. This can be done using conventional engineering determinations.

We then know the amount of additional length which flow line riser 24 will have if it is not to carry any appreciable anchoring load. We provide this extra length in the extra length caused by the buckling of the riser pipe. In other words, we cause the pipe to buckle sufficiently so that the length of the bent pipe exceeds the vertical distance from the anchor base 20 to the buoyancy means 14 by an amount equal to the maximum expected elongation of cables 16 and 18. This permits the cables to take all of the vertical load.

However, there is another load which we must consider and that is the lateral dynamic load on the riser pipes. Ordinarily, when only a riser pipe is used, the riser is subjected to a large axial tension to minimize such lateral motion and thus keep the dynamically induced stresses small. However, we make provisions for that in our system. We centralize the system. In other words, we situate the fiow line riser between two or more cable tethers 16 and 18 and centralize the cables and risers at a sufiicient number of points so that the system responds to lateral load as if the same tension were applied directly to the riser. However, the riser is not hearing sufficient tension to be considered a principal tether.

We place centralizers 34, 36 and 38 at special points along the flow line riser 24 so that they do not interfere with buckling. As will be seen, by knowing the desired shape of the buckled riser, we can calculate where the nodes occur. It is at these nodes that we place centralizers 34, 36 and 3-8. These centralizers 34, 36 and 38 are fixed to flow line riser 24 but are slideably attached to cables 16 and 18. This permits the centralizers to slide along the cables as necessary.

The lower ends of cables 16 and 18 can be connected to base template 20 in any desired manner. The lower end of flow line riser 24 can likewise be connected in any known manner, such as by use of an autoconnect, to well head 26. However, the upper end of the cables and the pipe are connected differently. Attention is directed to FIG. 2 which illustrates this means. This includes two vertical passages 16A and 18A within chamber -14 and through which cables 16 and 18 extend upwardly above the water level within chamber 14 to connect to a bulkhead in a known manner.

Flow line riser 24 extends upwardly through a vertical conductor 24A within chamber 14. Flow line riser 24 is connected to the base in any known manner. After such connection it is held in tension, that is, just so compression is removed from the lower end. The reference mark 43 is applied to flow line riser 24. The tension on the pipe is then slacked off so that mark 43 drops a distance AL, which as pointed out above is equal to the expected maximum elongation of the cables under maximum surges. The weight of the riser pipe itself is ordinarily sufiicient to obtain enough buckling or slack to obtain the entire length AL. The upper end of the riser pipe 24 is then fixed to the buoyancy means 14. FIG. 2 illustrates a convenient way of doing this. A large fiange 44 is welded or otherwise secured to the upper end of riser pipe 24. A split-ring shim 46 is then inserted between flange 44 and bulkhead 48. This shim 46 is of the proper vertical dimension to obtain the desired buckling of riser pipe 24. Flange 44 is held downwardly firmly against the shim by bolts 49.

FIG. 3 illustrates another embodiment for connecting the upper end of the riser pipe. Riser pipe 24 and cables 16 and 18 extend upwardly through passages in buoyancy means 14 in a manner similar to that of FIG. 2. Here, a flange 50 is attached to the upper end of pipe 24. Flange 50 has a vertical bore therethrough through which cables 16 and 18 pass. Locking means 52 and 54 on cables 16 and 18 are on the upper and lower side, respectively, of flange 50. With locking means 52 and 54 unlocked, the flow line riser 24 is slacked oif similarly as described above for FIG. 2. When riser 24 is lowered sufliciently, locking means 52 and 54 are locked to cables 16 and 18.

It will be recalled that, above, we stated that the riser pipe 24 should be centralized at its nodes when in the buckled form to the cables 16 and 18. Attention will now be directed to a form of mathematical analysis for determining the position of such nodes.

The problem of determining the position of nodes along a beam subjected to a uniformly varying axial tension, or compression, i.e., varying due to its own weight, has not been solved in closed form. However, the governing differential equation is d glij +H cos [6(s)]-V(s) sin [0( (1) vertical component. It is assumed the V(s) varies linearly with respect to length dV/ds=W (2) due to the unit weight of the member, W a constant.

Assuming that 6 is always small, the differential Equation 1 may be linearized, which in turn leads to an approximate solution of the form where 90:8/0', o'=3\/E1/ W and has unit of length, a and b are unknown coefiicients to be used in satisfying the boundary conditions, and Ai(x) and Bi(x) are the Airy functions. A suitable discussion of these functions, including tabulated values, is found on page 446 (Article 10.4) of Handbook of Mathematical Functions With Formulas, Graphs and Mathematical Tables, which is published by the U8. Department of Commerce, National Bureau of Standards, June 1964.

If it is assumed that the riser is fixed at both ends, then The eigen-value of the above equation ar is related to x by where D is the overall length of the riser and x and x are the non-dimensional coordinates of the lower and upper ends, respectively, of the riser.

There exists an infinite number of eigen value solutions to Equation 7, where all eigen values ar are negative. The smallest value of x (in magnitude) satisfying Equation 7 gives the solution for the first buckling mode, the next smallest for the second buckling mode, etc. The critical axial force at the upper end corresponding to each mode is obtained by V1=x1'0' W where a positive value of V indicates tension at the top. For V positive, the roots of (7) are approximately equal to the roots of Ai(x )=0 (10) If the riser is buckled in the second or higher order modes, then there is one or more points where the riser crosses the vertical axis, called nodes. The distance of these points above the bottom of the riser can be determined from t= ("lt o) where m are the roots of which are greater than ar x The following discussion explains how the above equations can be used to design the buckled riser.

The riser is centralized between two or more cables at a sufiicient number of points so as to minimize its lateral deflections. However, the centralizers are not to interfere With the desired buckling mode of the riser. Therefore, the centralizers are to be located only at the nodes of the buckled configuration.

First, the desired mode of buckling is selected, say the N mode. There will be N -1 nodes, or centralizers, each located a distance d, from the bottom, (i=1, 2 N 1), where d, is given in (11) above.

As the riser is fabricated atop the platform and then run while guided by the cables, the centralizers are located as discussed above. When the riser reaches the base plate and is attached, the tension at the top will be about equal to the buoyant weight of the riser and remains constant. Once attached, this tension may be decreased further to the value given by Equation 9 above, where buckling should occur. At this point the top of the riser can be displaced downward a significant amount without an appreciable decrease in tension; consequently, the riser is given slack. The amount of this slack should be slightly more than the anticipated vertical stretch in the cables.

While the above embodiments of the invention have been described with considerable detail, it is to be understood that various modifications of the device can be made without departing from the scope or spirit of the invention.

We claim:

1. A method of connecting a riser pipe from an underwater well to a buoyancy means which is connected to the bottom of a body of water by vertical tension members, variations in the vertical forces acting on said tension members causing a known maximum elongation therein, which comprises:

supporting said riser pipe along at least one of said tension members which includes connecting the lower end of said riser pipe to said underwater well;

slacking off support of the upper end of said riser pipe so that said riser pipe bends sufficiently so that the length of the bent riser pipe exceeds the vertical dimension from the buoyancy means to said well head by a distance at least equal to the said expected maximum elongation of said tension members;

attaching the upper end of said bent riser pipe to said buoyancy means.

2. A method as defined in claim 1 which includes:

determining the nodes along said bent riser pipe and slideably attaching said riser pipe to said tension member at each such node.

3. A method as defined in claim 1 which includes positioning said riser pipe between at least two tension members which are cables and laterally connecting said riser pipe to cables with centralizers which are fixed to either the cables or risers and slideably attached to the other.

4. A method of installing a riser pipe from an underwater well to a buoyancy means which is anchored to the bottom of a body of water by cables which comprises:

supporting the lower end of said riser pipe from said bottom;

buckling said riser pipe to provide axial flexibility;

tying said riser pipe to said cables to restrain lateral movement of said buckled riser pipe.

5. An apparatus for connecting a submerged well to a buoyancy means supported in a body of Water which comprises:

vertical tension means anchoring said buoyancy means to the bottom of said body of Water, said tension means having a maximum elongation for an expected maximum surge of vertical force on said buoyancy means;

a riser pipe extending from said well to said buoyancy means, said riser pipe being in a buckled configuration when said buoyancy means is in its neutral position, the length of said buckled configuration exceeding the length of the unbuckled riser pipe by an amount equal to the maximum elongation of said tension means;

means attaching the buckled riser pipe to said buoyancy means so that when said buoyancy means is in its 7 8 neutral position said riser pipe is in its buckled posi- References Cited UNITED STATES PATENTS 6. An apparatus as defined in claim 5 including centralizer means fixed to said riser pipe and slideably at- 2909359 10/1959 a at 1757 tached to said vertical tension means. 5 3'378'086 4/1968 Klvslld 175-7 7. An apparatus as defined in claim 5 in which said ten- 3413946 12/1968 Von Schultz 6146-5X sion means are at least two cables.

8. An apparatus as defined in claim 7 in which said TRYGVE BLIX Pnmary Exammer buckled riser pipe has points of nodes located along the U 5 G X R length thereof and including means fixed to said riser pipe 10 at said nodes and slideably attached to said cables so as to prevent lateral movement of said riser pipe. 

